Fans only add heat into the system if you have a temperature in their definition. I think this is the best way to include that effect vs the other methods that you describe. They are valid, but I don't think are the best fit. Regarding how much power, it isn't likely 12w. Consider that most of it probably goes to turning the fan. Go with a worst case input temp plus a couple degs maybe for the energy that is converted to heat.

You can bound the heat that the fan dissipates. Moving some mass rate x [kg/s] of air from a dead stop to some velocity v can be converted to a power P (x*v^2/2, I think.). Then the heat dissipated will be volts*amps - P. This heat will be an upper bound, because getting the air started through a turbulent fan actually involves a lot of non-conservative work. I'd be curious whether this is useful or whether the bounds would be so conservative it doesn't really help.